How neural circuits control complex behaviors is a fundamental problem in neuroscience. Revolutionary new tools, such as optogenetics, and the ability to express such tools in molecularly defined neuronal cell types, have transformed our ability to dissect neural function in genetically tractable model organisms such as the mouse. However, a gap remains between the ability to mark, map and manipulate neuronal populations defined by Cre driver transgenic mouse lines, and potential anatomic and functional heterogeneity within those populations. This gap limits our ability to dissect neural circuit function at an appropriate level of cellular specificity. To fill this gap, combinatorial approache will be developed to express optogenetic effectors and other genetically encoded tools in neuronal subpopulations defined by the intersection of their molecular identity, and their activity or connectional specificity. The long-term goal is to elucidate the neural circuits that control decisions between complex, innate social behaviors controlled by the hypothalamus, amygdala and other limbic structures. The overall objective of this application is to develop new methods for identifying and genetically targeting heterogeneous neural cell types contained within populations defined by the expression of a given Cre driver. Proof-of-principle for these methods will be obtained by dissecting hypothalamic circuits that govern behavioral decisions, using innate social behaviors as a robust read-out of targeted optogenetic manipulations. The central objective of this proposal is to develop intersectional methods that combine the expression of a specific Cre driver with orthogonal genetic methods that mark neurons based on their activity during particular behaviors, or their connectivity, in a contingent (logical 'AND') manner. The rationale for this research is that solving this general problem is essential to making forward progress in mapping the circuitry that governs complex behaviors. In this proposal, methods will be developed to mark neurons based on their expression of a specific Cre driver and: their activity during different innate behaviors (Aim 1); their projections to a specific target (Aim 2); and their presynaptic inputs and behavior-specific activation (Aim 3). The contribution will be to create new intersectional tools for marking and manipulating specific neuronal subpopulations that will be generally applicable, and to use them to gain new insights into the functional organization of hypothalamic circuits controlling behavioral decision-making. This contribution is significant because it will create new tools that allow dissection of neural circuit function at an unprecedented level of cellular specificity, and shed new light on an important problem in the neural control of complex behaviors. The approach is innovative, because it will provide intersectional approaches to genetically targeting specific neuronal cell types that have not yet been implemented in mice. The work proposed in this application will, therefore, both advance knowledge in this specific field and, by developing and disseminating novel methods, will also advance knowledge in other fields of neuroscience.